I am running one of Arjen Helder's TA2020s straight from the sound card on my PC. It's built up and works great off a battery or wal wart, but I'd really love to drive it using the computer PSU (so it turns on/off with the PC and there are fewer parts). Problem is, there is lots of noise through the spreakers when it's driven from the SMPS - and the noise varies in pitch/ volume depending on what the rest of the computer is doing.

I don't think it's easy to clean up the power from a dirty SMPS. Probably the power is polluted by the pc itself too.

However, if you're only interested in switching the TA2020's supply on an off simulteneoulsy with the computer and don't mind using a separate PSU, why not use the computer's 12 V to power a relay that switches (external) power for the TA2020 on/off?